The present invention relates to a multi-color thermo-sensitive recording material and a process for preparation thereof.
The thermo-sensitive recording material is a recording material comprising a support member, for example, paper, and a thermo-sensitive layer capable of forming a color upon application of heat, which layer is formed on the support member. For the application of heat, for example, a thermal printer with a thermal head is employed. The multi-color thermo-sensitive recording material comprises a support member and at least two thermo-sensitive layers formed on the support member. Each of the thermo-sensitive layers is capable of forming a different color at a different temperature. Usually, a lower layer is capable of forming a color at a high temperature, while an upper layer is capable of forming another color at a low temperature.
When heat is applied to such a multi-color thermo-sensitive recording material at two different temperatures, images are formed in different colors. When heat is applied to the multi-color thermo-sensitive recording material at a comparatively low temperature, a blue color, for example, is formed, while when the recording material is heated at a comparatively high temperature, a red color, for example, is formed.
When such colors are formed, using the multi-color thermo-sensitive recording material, if the recording material is heated at a low temperature, the color to be formed at the low temperature is normally formed. However, when the recording material is heated at a high temperature, two colors, that is, the two colors to be formed at a low temperature and a high temperature, respectively are both formed and mixed. In order to eliminate such a shortcoming, it has been proposed to use several decoloring agents capable of forming the respective colors separately at a low temperature and a high temperature. As the decoloring agents, solid alcohols, polyether, polyethylene glycol, and quanidine derivatives are employed in such a manner that any of these decoloring agents is inserted between the thermo-sensitive layers.
The multi-color thermo-sensitive recording material is prepared by forming a high temperature thermo-sensitive coloring layer, a decoloring layer and then a low temperature thermo-sensitive coloring layer one over the other on a support member. The inventors of the present invention produced the multi-color thermo-sensitive recording materials of this type by a continuous coating machine after the investigation of an experimental scale and were confronted with a problem that, in the initial step of coating the low temperature thermo-sensitive coloring layer, the thermo-sensitive recording materials were produced with the desired properties, but as the coating process proceeded, the coloring density of the low temperature thermo-sensitive coloring layer was gradually lowered so that the properties of the multi-color thermo-sensitive recording materials produced became nonuniform. In the course of the investigation of this problem, the inventors discovered that part of the surface of the decoloring layer was dissolved into the low temperature thermo-sensitive coloring layer during the coating thereof and the materials of the dissolved decoloring layer built up in a container of the low temperature thermo-sensitive coloring layer materials, so that when the multi-color thermo-sensitive recording material is produced, using the low temperature thermo-sensitive coloring layer liquid contaminated with the decoloring layer materials, the products produced in the initial coating process and those produced thereafter differ in the coloring density.